The 75 kDa neurotrophin receptor (NTR) cooperates with tyrosine kinase receptors in regulating neuronal differentiation and mediates programmed cell death in neurons and other cells deprived of neurotrophic survival factors. After more than a decade of work on this membrane protein, how it transmits signals for cell survival or death remains unknown. The neurotrophin receptor was the first discovered member of a family of cytokine/growth factor receptors that mediate or prevent programmed cell death. Homologous to the 55 kDa tumor necrosis factor receptor (TNFR-I) and Fas/Apo-I, the neurotrophin receptor has a ``death motif'' near the C-terminus of its intracellular domain. However, in contrast to the other two death receptors in the family (Fas and TNFR-I), NTR induces apoptosis in the coincident absence of its ligand and ligands of tyrosine kinase receptors. Multiple signals from serum factors and neurotrophins are thus integrated to control NTR activity. Proposed preliminary research applies methods of structural and computational biology to the problem of identifying binding sites of regulatory and signal transducing proteins. Molecular graphics software provided by the Computer Graphics Laboratory will be used to carry out the analysis. The goal of these studies is to define targets for biochemical and structural analysis of events in signaling through the receptor. The objectives of this proposal are 1) To map the intracellular domain of NTR by structure-based multiple alignment with other members of the receptor family. 2) To find variant motifs and potential signaling partners through profile analysis and database searches. 3) To evaluate secondary and tertiary structural features encoded by motifs using secondary structure analysis and molecular modeling. 4) To characterize potential interfaces with signaling or regulatory proteins by protein-protein docking with drug design software. Results from proposed computational studies will guide experimental work to rapidly identify signaling and regulatory partners for NTR. Preliminary data will address the possibility that the neurotrophin receptor induces cell death through heterotrimeric G-protein signaling molecules. It will also be used to evaluate serine/threonine phosphorylation as a mechanism by which the apoptotic activity of NTR is down-regulated. An important target of this effort is to predict binding sites of regulatory proteins. In addition, methodology will be established that can be applied to other family members, and potentially to all non-catalytic receptors that signal through protein-protein interactions. Proposed studies thus integrate molecular graphics tools and computational chemistry to develop a picture of signaling through the neurotrophin receptor.